It is known to use frequency converters comprising a voltage link for open-loop and closed-loop control of electric drives. Corresponding control circuits first generate a direct voltage from a mains voltage, which is in most cases a first three-phase voltage, via a B6 diode bridge or via a feed-in converter. The generated direct voltage may e.g. amount to 700 V. By means of the control circuit, the direct voltage is subsequently converted into a second three-phase voltage via a converter, the second three-phase voltage comprising an amplitude and a frequency which are adjustable to a certain extent. By varying the amplitude and frequency, torque and rotational speed of the electric motor provided with the second three-phase voltage may be set.
The converter for generating the second three-phase voltage usually comprises six power-electronic switches, typically IGBT switches. The power-electronic switches are e.g. accessed by means of pulse-width modulation according to the space-vector-modulation method in order to generate the second three-phase voltage comprising the desired amplitude and frequency.
The power-electronic IGBT switches do not operate in an ideal manner. If the switches are enabled, conduction losses accumulate. Moreover, switching losses accumulate during the switching of the switches.
If the power-electronic switches are accessed with a high switching frequency, short response times and a low current-ripple factor are the result, but also high switching losses. If the power-electronic switches are accessed with a low switching frequency, the switching losses decrease, thus increasing efficiency. However, a lower switching frequency is attended by declined response times and a higher current-ripple factor.
Another option would be to directly convert the first three-phase voltage provided as mains voltage into the second three-phase voltage for supplying the electric motor by means of a three-phase autotransformer having an adjustable amplitude. However, such three-phase autotransformers have a considerable size and comparatively low efficiency.
Instead of a converter comprising IGBT switches, a converter having MOSFET switching transistors could be used, as well. Thereby, small-range drives having a power of approximately 2 kW could be operated. Suitable MOSFET switching transistors are available with block voltages ranging up to 600 V. By means of these transistors, frequency converters with voltage links of up to approximately 400 Vdc may be configured.
The efficiency of such MOSFET converters may be more than 99%. Due to the low losses, the MOSFET converter may be configured in a compact manner, thus allowing for integration of the MOSFET converter into the motor.
A motor-integrated MOSFET converter may be provided with power by means of a direct voltage of about 350 Vdc. This supply voltage of 350 Vdc may e.g. be generated from the direct voltage yielded from the mains voltage by means of a step-down converter or, respectively, one-quadrant chopper. The power semiconductors and the inductively of the step-down converter must thereby be adjusted to the largest possible peak current. This again results in power losses of the step-down converter, reducing the overall efficiency of the entire drive system.
Instead of a step-down converter or a one-quadrant chopper, use might also be made of a two-quadrant chopper. It allows for feeding back the power stored in the drive maintaining a relatively high efficiency.